The following briefly introduces the dyeing and printing properties of common dyes for pure cotton fabrics.
1. Direct dyes
Direct dyes are soluble in water and can be directly dyed on cotton fibers in neutral or weak alkaline solutions. Adding neutral electrolytes such as salt during dyeing can increase the amount of impurities on the dye. Direct dyes have complete chromatograms, simple dyeing methods and relatively cheap prices. They were once the main beam materials of cotton fabrics. However, the dyeing mat of this kind of dye is not good enough, and it needs to be fixed in the post-processing.
2. Reactive Dyes
Reactive dyes refer to soluble dyes containing reactive groups. Under appropriate conditions, it can chemically combine with cellulose fibers and protein fibers, greatly improving the color fastness of dyed objects to washing and soaping. At the same time, it also has the advantages of simple dyeing and printing, bright color, uniform color spectrum and cheaper price. However, generally reactive dyes have a low fixation rate and are easily hydrolyzed in alkaline solutions, resulting in floating color. Therefore, the method of dyeing first and then fixing the color of the fabric is adopted to improve the fixing rate of the dye. For printing, one-phase or two-phase printing should be selected according to the different types of dyes. There are also some reactive dyes with poor color fastness to chlorine bleaching and color fastness to weather.
3. Vat dyes
Vat dyes are insoluble in water and cannot be dyed directly. It needs to be reduced to leuco in the strong alkaline reducing solution of caustic soda and hydrosulfite, and the cotton fiber is dyed by the leuco. After oxidation, it is restored to insoluble dye and fixed to the fiber. The chromatogram of vat dyes is complete, the color is bright, and the color fastness to sunlight and soaping are all good. However, due to the high price, some yellow and orange varieties have the effect of photosensitive brittleness, so their application is limited. When dyeing or printing, it is necessary to strengthen the post-soap treatment to remove floating color and improve the brightness.
4. Soluble Vat Dyes
Soluble vat dyes are derivatives of vat dyes, which are soluble in water. After being dyed on fibers, they need to be oxidized in an acid solution to hydrolyze and oxidize the dyes and revert to insoluble vat dyes for dyeing on fibers. The dyeing process of soluble vat dyes is relatively simple, the dyeing is more uniform, and the dyeing fastness is high. However, this dye is expensive and is mainly used for dyeing and printing light and medium-color cotton fabrics.
5.
Sulfur dyes
Sulfur dyes are insoluble in water, but can be dissolved in sodium sulfide solution and reduced to leuco. After the leuco is absorbed by cotton fibers, it is oxidized into insoluble dyes and fixed on cotton fibers. Sulfur dyes are mainly used for the dyeing of dark-colored products of cotton fibers. The degree of washing and sun exposure is good, the dyeing method is simple and the price is cheap, but the color fastness to rubbing is poor and the color is not bright enough. Some dyes (such as sulfur black) will make the fibers brittle during the storage process of the fabric, so anti-brittle treatment is required after dyeing. Yellow and orange sulfur dyes often have a photosensitive brittle effect on fibers.
6. Insoluble azo dyes
Insoluble azo dyes are composed of a primer (chromophore) and a developer (chromophore) in the middle of two dyes. When dyeing, the fabric is first primed with a caustic soda solution of chromophore, and then developed with a diazonium solution of a chromophore. The chromophore on the fabric and the diazo compound of the chromophore are coupled to form insoluble azo dyes, which are fixed on the fabric. superior. Since ice is required for the diazotization reaction of the chromophore, insoluble azo dyes are also called ice dyes. This type of dye has bright color, good color fastness to soaping, and good color fastness to sunlight, but poor color fastness to rubbing, and even worse fastness to wet rubbing.
7. Stabilize insoluble azo dyes
Stabilized insoluble azo dyes are mixtures of sodium chromate and temporarily stabilized diazo compounds. Under normal circumstances, the two do not undergo a coupling reaction. After a certain treatment after printing, the temporarily stable diazo compound is converted into an active diazo compound, which is coupled with chromophore to form a non-deep azo dye. These dyes are mainly used to print small patterns.